A common method of recovering argon from air is to use a double column distillation system consisting of a higher pressure column and lower pressure column thermally linked with a reboiler/condenser. This method also includes the use of a sidearm rectifier column attached to the lower pressure column. The oxygen product is withdrawn from the bottom of the lower pressure column and at least one nitrogen-enriched stream is withdrawn from the top of the lower pressure column. The relative volatilities of nitrogen, argon, and oxygen force argon to accumulate in an intermediate section of the lower pressure column. Thus, to produce an argon-enriched product, a portion of the vapor rising through the lower pressure column is withdrawn from this intermediate location and passed to a sidearm column.
The portion passed to the sidearm column generally contains between 3 mole % and 25 mole % argon, traces of nitrogen, and balance oxygen. This first argon-enriched stream is rectified in the sidearm column to produce a second argon-enriched stream substantially purified of oxygen. Typically, this second argon-enriched stream is withdrawn from the top of the sidearm column with an oxygen content ranging from less than 1 ppm to 3 mole % oxygen. The rectification is achieved by providing liquid reflux to the sidearm column via a condenser located at its top.
The sidearm column need not be contained in only one vessel but can be split into more than one vessel. Each vessel is connected to the next in the series by a vapor and liquid stream from its top to the bottom of the next vessel. The bottom of the first vessel is attached to the lower pressure column and the top of the last vessel contains a condenser as described above. Typical practice is to limit the total height of the cryogenic system when the sidearm column becomes too tall by splitting it into two or more vessels.
Due to the high value of argon, it is important to maximize the production of the enriched argon product stream. To maintain a high argon product flow rate, it is important to limit the nitrogen content in the vapor feed stream to the sidearm column. If the nitrogen content of the feed stream increases, the nitrogen tends to accumulate in the condenser located at the top of the sidearm column. Such an accumulation of nitrogen decreases the condensing capability of the condenser which in turn lowers the performance of the sidearm column by decreasing the amount of vapor that can be fed to the column.
U.S. Pat. No. 4,784,677, U.S. Pat. No. 4,842,625, and U.S. Pat. No. 5,448,893 all disclose processes for maximizing the recovery of argon from a cryogenic air separation system. Patentees in each instance disclose ways to operate at the upper limit of the nitrogen flow rate to the sidearm column during normal steady-state operation. None, however, address the problem of how to operate at the lower limit of the argon concentration in the oxygen product stream in the presence of increased argon accumulation due to process or product rate transients.
The oxygen product, which is withdrawn from the bottom of the lower pressure column, generally contains between 0.1 mole % and 2 mole % argon when an argon enriched product is co-produced. The oxygen product purity is usually allowed to vary in a dead-band range above the required customer purity specifications and is only controlled when it approaches the minimum purity range, defined by customer need. Thus, even if the oxygen product stream is operating at or above the minimum purity required, argon is escaping from the system as an impurity in the oxygen product stream.
It is known to recover this potential argon product from the oxygen product stream by increasing the enriched-argon stream flow rate out of the sidearm column while maintaining appropriate nitrogen levels. This method causes the increased recovery of argon as product. Conditions can exist in the process, however, that render the above method ineffective. In such circumstances, argon in the oxygen product stream cannot be controlled by increasing the enriched-argon stream flow rate leaving the sidearm column because the enriched-argon product stream is either below the required purity or the flow rate can otherwise not be increased. In such circumstances, it is either not desirable or not possible to maintain the desired overall argon production by increasing the enriched-argon stream flow rate out of the sidearm column.
Several possible process conditions can exist that will result in not being able to control the argon lost from the system in the oxygen product stream by increasing the enriched argon product stream from the sidearm column. These are:
(1) The sidearm column is being started-up or restarted. To quickly re-establish the argon purity, argon-rich liquid inventory is re-introduced to the system while the sidearm column is operated at total reflux without a top product stream as disclosed by U.S. Pat. No. 5,505,051, and German Patent 34 36 897; PA0 (2) The customer demand for argon either is constant or has been curtailed while the oxygen and nitrogen requirements have been increased; or PA0 (3) The sidearm column, or the entire air separation unit, is undergoing transient operation such as that caused by time-of-day contracts for which the production rates see large daily changes due to electrical power costs.
An improved process would allow for control of overall argon production without manipulating the flow of the argon-enriched stream from the sidearm column.